This invention relates in general to well perforators and more specifically, to improvements in lined shaped charge means for perforating well casings and the surrounding earth formations.
Explosive shaped charge well perforating devices are often used in the perforating of oil well casing and the surrounding earth formations. In the typical embodiment, a plurality of shaped charges are mounted in a fluid-tight, cylindrical, metal housing or on an elongated bar member which is adapted to traverse the borehole to be perforated. The shaped charges are mounted in the housing or on the bar member at longitudinally spaced intervals, with their axis of perforation directed laterally thereof.
The shaped charge most common in well perforating is a conical shaped charge. A conical shaped charge consists of an explosive material having a substantially conical cavity formed in the front face. A metal liner material covers the face of the cavity. Upon detonation the shape of the explosive cavity focuses and propagates a progressive wave front against the outside surface of the metal liner. At the pressures generated, the metal acts as a fluid. Metal in atomized form is squirted into a focused jet stream. The resultant focusing force moves particles forward and relatively backward to form a jet which lengthens as the wave front advances from apex to base of the conical cavity. The extreme high pressure, particle-laden jet stream breaks down and moves aside any material upon which it impinges. Penetration of such material is a result of the amount of pressure and the kinetic energy in the jet stream. The greater the length of the jet stream, the greater the depth of penetration.
It is highly desirable that a perforation in the formation be as deep as possible so that fluid in the formation may have access to the borehole. One way to increase the length of the jet stream is to deepen the conical cavity, that is, to decrease the apex angle of the cavity. However, if the angle is decreased below 25.degree., the Monroe effect disappears and the charge explodes like a petard. Another method of increasing the length of the jet stream is to build a charge of a larger caliber. However, increasing the caliber presents many drawbacks. As the caliber of the charge increases, the size and weight of the perforating instrument housing the charge has to be correspondingly increased. The restrictive diameter of an oil well limits the size of the perforating instrument operated therein and thus limits the caliber of the charge.
These and other disadvantages are overcome with the present invention by providing a method and apparatus for perforating a well casing and the surrounding formations using a lined shaped charge having a jet stream of increased length whereby the depth of penetration is substantially increased without increasing the caliber of the lined shaped charge.